src/svgtiny_css.c: change lwc_string_destroy -> lwc_string_unref

src/svgtiny_css.c: change lwc_string_destroy -> lwc_string_unref

In the node_is_lang() callback, we're interning "lang" and later
destroying it. But "lang" is already interned at that point (in
libcss), so we are destroying a copy that doesn't belong to us. It
should be unref'd instead. Valgrind detected the error. Thanks to
Michael Drake for the pointer.